In the rubber industry, and more particularly in the tyre industry, it is known to add reinforcement fillers to the elastomeric compositions in order to improve the mechanical properties and the abrasion resistance of the elastomeric materials obtained therefrom by vulcanisation.
Due to its high reinforcing power, carbon black is the most commonly used filler. However, it imparts a strong hysteresis to the articles, i.e. it increases the dissipated heat under dynamic conditions. In tyres, this results in the undesired increase of the rolling resistance, and overall in higher fuel consumption, in the production of more polluting emissions and higher transport costs.
Currently, the majority of vehicle manufacturers increasingly require their suppliers to develop low rolling resistance tyres to reduce consumption.
In order to decrease the hysteresis of elastomeric materials, it is not decisive to use small amounts of carbon black and/or a carbon black with reduced surface area, as doing so compromises the reinforcement activity, thereby worsening the static mechanical properties and the resistance abrasion of the final product.
An improvement in this sense was achieved by the use of the so-called “white” reinforcement fillers, such as chalk, talc, kaolin, bentonite, titanium dioxide and especially silica, fillers which may partially or totally replace the carbon black in elastomeric materials and impart a lower hysteresis to them while maintaining sufficient reinforcement.
However, a need remains to further reduce the rolling resistance of the tyres, and thus to identify new fillers that allow a further improvement in the balance between hysteresis and reinforcement of materials.
In fact, the hysteresis of the elastomeric material filled with silica still remains too high for certain specific applications, for example in tyres with ultra-low rolling resistance (ULRR) or in self-supporting tyres (run-flat) in which significantly lower heat dispersion and rolling resistance are instead required. The elastomeric materials filled with silica and/or silicates do not always show sufficient performance when incorporated in the components of the tyre subjected to strong stress, such as the tread, under-layer, anti-abrasive elongated element (or anti-abrasive band), sidewall, inner layers or sidewall insert, such as the sidewall insert of a self-supporting tyre.
Moreover, a problem of fillers in general, in particular silica, is represented by the fact that under dynamic conditions, i.e. when the elastomeric material filled with silica and vulcanised is stressed in the tyre in use, a partial breakdown of the dispersed filler can occur that adversely affects the mechanical properties. This phenomenon occurs with a reduction of the dynamic module that is more marked when the deformation to which the elastomeric material is subjected is higher. In practical terms, just when the tyre is most stressed and then just when the elastomeric material should show the best mechanical performance, the reinforcing effect of the filler is however lacking. This phenomenon is known as Payne effect. Fillers based on silicate fibres, while unexpectedly improving the drivability of cars subjected to high operating speeds and/or extreme driving conditions, seem to not overcome the drawbacks of silica in terms of excessive rolling resistance and less support at higher deformations.
In this regard, document WO2012164433A1 on behalf of the applicant describes tyres which have improved performance during use in extreme conditions, in particular an improved driving stability, especially at the rear side of the vehicle. At least one layer of elastomeric material is applied in these tyres, in a radially inner position with respect to the tread band, comprising fibres of nanometric size consisting of magnesium and/or aluminium silicates, in particular sepiolite. The elastomeric material, which is considerably reinforced by the sepiolite fibres, however shows a strong decrease of the dynamic shear modulus with increasing dynamic deformation (Payne effect) and a higher hysteresis compared with the material filled with silica.
Some general studies are known from the literature which describe acid treatment processes of silicate fibres, in particular of sepiolite. Depending on the more or less drastic conditions applied, such treatments can lead to the complete removal of the ions interspersed among the silicates and conversion of silicates in unstructured amorphous silica (SilSep) or to the partial removal of the ions and conversion of the silicates into silica only at a surface level, while preserving the needle-shaped morphology of the fibres.
For example, article “Novel anhydrous unfolded structure by heating of acid pre-treated sepiolite”, Valentin, J. L., et al. Applied Clay Science, 2007, 36(4), 245-255 and article “The role of magnesium on the stability of crystalline sepiolite structure” Esteban-Cubillo, A. et al., Journal of the European Ceramic Society, 2008, 28(9), 1763-1768, describe the effects of the partial or total removal of magnesium on the morphology of sepiolite.
To the Applicant's knowledge, there are no hints in the prior art to advantageously use, as additional fillers in elastomeric materials for tyres, needle-shaped silicate fibres acid-modified in mild conditions and with only partial removal of magnesium. In fact, article “Effect of the Textural Characteristics of the New Silicas on the Dynamic Properties of Styrene-Butadiene Rubber (SBR) Vulcanizates” Polymer Composites, (June 1988), vol. 9, n. 3, 204-208 describes a study in which sepiolite fibres subjected to treatments with concentrated acid (nitric ac. 6N) to yield silica of different surface area, are incorporated in elastomeric materials and then evaluated in terms of dynamic properties. The article does not suggest the partial removal of magnesium or the preservation of the needle shape of the fibres.
In the article “Preparation of Silica by Acid Dissolution of sepiolite and Study of its reinforcing effect in Elastomers”, Die Angewandte Makrom Chemie (1982), 103, 51-60, the authors describe the preparation of materials having a different magnesium content by treatment of sepiolite fibres with nitric acid, at variable temperatures and times. The study reports the use, in elastomeric materials for tyres, of fully extracted sepiolite described as an amorphous silica with a high surface area comprising less than 1% residual Mg, which has completely lost the crystalline order. This silica obtained by complete extraction of Mg is presented as a potential low-cost alternative to the commercial precipitated amorphous silica. The article reports only data about static mechanical properties related to mixtures filled with silica obtained by exhaustive acid treatment.
In conclusion, the relevant publications related to the tyre industry known to the Applicant teach drastic treatments of sepiolite fibres—with acid so concentrated or conditions so drastic as to completely subtract the magnesium and/or heavily modify the crystallinity and morphology of the fibres—and the subsequent incorporation of the high surface area silica thus obtained in elastomeric materials.